Many operations require the dissolution of gold, for example the isolation of gold metal from ores containing small amounts of the metal, the recovery of gold from gold plated materials, and the preparation of soluble gold metal compounds for applications such as catalyst manufacture. However, gold metal is notable for its resistance to dissolution and chemical reaction. For example, gold will dissolve in an aqueous solution only if the solution contains a good ligand or bonding agent for gold plus an oxidizing agent, neither ingredient alone being sufficient. This combination of ligand and oxidant is referred to as a lixiviant system.
For gold, a number of lixiviant systems have been proposed and used over the past century, primarily for mining applications. The most widely used system is a combination of sodium cyanide as ligand together with air (oxygen) as oxidant, primarily because of economy and process simplicity. Other ligand/oxidant systems of importance are thiourea and thiocyanate with ferric ion; sodium thiosulfate with air, copper(II) sulfate and ammonia; sodium chloride with sodium hypochlorite; sodium bromide with bromine; and a number of other systems. The chemistry of these and other alternative lixiviant systems is described in a review article by J. B. Hiskey and V. P. Atluri, Mineral Processing and Extractive Metallurgy Review, 4, 95-134 (1988). The choice of a particular lixiviant system depends on a variety of factors including ingredient cost, safety, environmental concerns, and corrosion of equipment. For mining operations, the ease of separating the impurities associated with gold in the ore is frequently the primary factor in choosing one particular lixiviant system over another.
Certain divalent metal cations such as lead, mercury, thallium and bismuth are known to accelerate gold dissolution in cyanide solutions. The mechanism of this action is not known, although depolarization of the gold surface and prevention of passivation of the gold surface have been suggested. The use of these metals is undesirable. If they are recovered with the gold, expensive purification steps for the gold may then be required. If any portion of these highly toxic metal ions becomes trapped with the tailings from the ore processing, they create a severe, long-lasting environmental risk.
An improvement to any of the generally used lixiviant systems would have high value, particularly if it were broadly applicable to a wide variety of these systems. This improvement could result in a higher yield of dissolved gold under standard dissolution conditions, a shorter time cycle for this step, less severe operating conditions, the use of smaller amounts of lixiviant ingredients, or in still other ways which reduce cost, improve safety, or increase gold recovery. The prior art does not disclose a method of enhancing the operation of the generally used lixiviant systems without the use of poisonous metal compounds, nor one that is broadly applicable.
U.S. Pat. No. 3,597,290 discloses a method of chemically dissolving metals and particularly describes the etching of copper with acidified hydrogen peroxide. The method uses a solution containing a strong organic acid or a mineral acid (excluding hydrohalic acids) plus hydrogen peroxide and a saturated lower aliphatic alcohol. Optionally, this system may contain a catalytic amount of the salt of a metal with a lower oxidation potential than the metal being dissolved to enhance the latter's dissolution, the catalytic materials including the salts of metals such as silver, mercury, palladium, gold, and platinum. Optionally, this system may also contain a nitrogen compound having at least one bonding site to copper, such as urea, pyridine, amines and acid amides. In contrast to the present invention, the '290 patent does not teach a method of enhancing the dissolution of gold or other metals, but rather proposes the use of catalytic amounts of certain noble metal salts to enhance the dissolution of other metals with a higher oxidation potential such as copper. The nitrogen compounds mentioned are optional rather than essential, and are utilized as bonding agents to copper to inhibit the undercutting of the copper during a selective etching process.
Polish Patent PL 130,801 discloses a process for the recovery of silver and copper from sulfidic ores by using a mixture of pyridine and its hydrochloride to dissolve the minerals containing these metal ions, filtering off the inert material, adding water, extracting with chloroform to remove the silver chloride and pyridine, and recovering the copper from the aqueous phase by electrolysis. This process is similar to many examples in the literature in which nitrogen heterocyclic compounds are used to extract metal complexes into an organic solvent. The '801 patent does not teach a method to enhance the operation of a gold lixiviant system containing a ligand and oxidant by adding catalytic amounts of an aromatic heterocyclic compound containing nitrogen or sulfur in the ring. Rather it teaches the use of large amounts of pyridine and pyridine hydrochloride to dissolve silver and copper sulfide ores and effect the separation of copper from silver ions by extraction of the silver and pyridine into a chloroform solution.
U.S. Pat. No. 5,169,503 discloses a process for extracting metal values from ores using a lixiviant system comprising a chloride salt, a hypochlorite salt and optionally cyanuric acid, a nitrogen-containing heterocyclic compound. The cyanuric acid is used to prolong the life of the hypochlorite solution under acidic conditions, even though it is stated to actually decrease the reaction rate between the metals being dissolved and the chlorine-containing compounds.
Numerous examples in the literature disclose the use of nitrogen-containing heterocyclic aromatic compounds in combination with soluble gold compounds for a variety of purposes. U.S. Pat. No. 4,654,145, Japanese Patent 01111824; Z. Zhang and W. Gan, Huaxue Shiji, 137-139 (1982); and M. Iqbal and M. Ejaz, Radiochim. Acta, 22, 37-39 (1975). For example, a common application is the recovery of gold from aqueous solutions of their salts by solvent extraction. These nitrogen-containing aromatic heterocyclic compounds are also used in the preparation of resins to sequester soluble metal species from solution and are often used to improve electrodeposition of gold by allowing the use of a high current density without loss of current efficiency. In all these applications, the heterocyclic aromatic compounds are reacting with soluble metal ions and play no role in the dissolution of the metal; in fact in the case of electrodeposition the process is just the reverse, removing the metal from its solution.
Accordingly, there is a need for a method to enhance the operation of lixiviant systems for the dissolution of gold metal that will increase the speed of dissolution, allow greater metal recovery, use less chemicals or allow the use of Milder conditions. In particular, there is a need for a catalytic method of enhancing such lixiviant systems which is broadly applicable to a number of systems and which does not require the use of poisonous metal salts.